A SwiftNIO based implementation of WebSocket for Kitura

What's New



What's new

  • Major fixes related to compression context takeover (#26 and #28)
  • A memory leak fixed (#33)
  • WebSocket compression documentation added (#37)
  • Don't log errors when closed channels detected (#36)


APIDoc Build Status - Master macOS Linux Apache 2 Slack Status


Kitura-WebSocket-NIO is a swift-nio based immplementation of the Kitura-WebSocket API. It provides Kitura based servers the ability to receive and send messages to clients using the WebSocket protocol (RFC 6455). It is compatible with a variety of WebSocket clients, including:

  • The built-in WebSocket support in the Chrome, FireFox, and Safari browsers.
  • The NPM websocket package.

Kitura-WebSocket-NIO supports version thirteen of the WebSocket protocol.

Both the WS and WSS (SSL/TLS secured WS) protocols are supported by Kitura-WebSocket. To enable WSS set up your Kitura based server for SSL/TLS support. See the tutorial Enabling SSL/TLS on your Kitura server on for details.

Kitura-WebSocket-NIO also supports WebSocket compression using the permessage-deflate algorithm, adhering to RFC 7692 as much as possible.

Table of Contents


Add dependencies

Add the Kitura-WebSocket-NIO package to the dependencies within your application’s Package.swift file. Substitute "x.x.x" with the latest Kitura-WebSocket-NIO release.

.package(url: "", from: "x.x.x")

Add Kitura-WebSocket to your target's dependencies:

.target(name: "example", dependencies: ["Kitura-WebSocket"]),

Import package

import KituraWebSocket

API Overview

The following is an overview of the Kitura-WebSocket APIs, for more information see the API reference.

When using the WebSocket protocol, clients connect to WebSocket services running on a particular server. WebSocket services are identified on a particular server via a path. This path is sent in the upgrade request used to upgrade a connection from HTTP 1.1 to WebSocket.

The Kitura-WebSocket API reflects that interaction using the class WebSocketConnection which represents a WebSocket client's connection to a service and the protocol WebSocketService which is implemented by classes that are WebSocket services.

A specific WebSocketConnection object is connected to a specific WebSocketService instance. On the other hand a specific WebSocketService instance is connected to many WebSocketConnection objects.


The WebSocketConnection class provides:

  • Functions to send text and binary messages to the client.

    WebSocketConnection.send(message: Data)
    WebSocketConnection.send(message: String)
  • Functions to close the connection gracefully and forcefully.

    WebSocketConnection.close(reason: WebSocketCloseReasonCode?=nil, description: String?=nil)
    WebSocketConnection.drop(reason: WebSocketCloseReasonCode?=nil, description: String?=nil)

    In both close() and drop(), the WebSocketCloseReasonCode enum provides the standard WebSocket close reason codes with the ability to specify application specific ones.

  • A unique identifier that can be used to help manage the collection of WebSocketConnection objects connected to a WebSocketService.

    id: String


The WebSocketService protocol enables Kitura-WebSocket to notify a WebSocket service about a set of events that occur. These events include:

  • A client has connected to the WebSocketService.
connected(connection: WebSocketConnection)
  • A client has disconnected from the WebSocketService.
disconnected(connection: WebSocketConnection, reason: WebSocketCloseReasonCode)

The reason parameter contains the reason code associated with the client disconnecting. It may come from either a close command sent by the client, or be determined by Kitura-WebSocket if the connection's socket suddenly was closed.

  • A binary message was received from a client.
received(message: Data, from: WebSocketConnection)

The message parameter contains the bytes of the message in the form of a Data struct.

  • A text message was received from a client.
received(message: String, from: WebSocketConnection)

The message parameter contains the message in the form of a String.


Classes which implement the WebSocketService protocol are registered with the server as follows:

WebSocket.register(service: WebSocketService, onPath: String)

This function is passed the WebSocketService being registered along with the path it is being registered on.

A registered WebSocketService can be unregistered from the server as follows:

WebSocket.unregister(path: String)

This function is passed the path which the WebSocketService being unregistered was registered on.

Autobahn TestSuite

Kitura-WebSocket-NIO fully complies to the Autobahn Testsuite for web sockets. To create an echo server and run this test suite against it, follow the instructions here.

Example - Simple

This example is a simplistic chat service to demonstrate how to use the Kitura-WebSocket APIs. The server side is written in Swift using Kitura-WebSocket and the client side is written in JavaScript using Node.js and the websocket NPM package. The instructions below show you how to create the files for both the server and client and then how to compile and run the application.


In order to run the client one must have Node.js installed.

The server

The server keeps track of the clients that have connected to it and echoes all text messages sent to it to all of the clients that have connected to it, with the exception of the client that sent the message.

You will need to create the server's directory structure to be something like this:

├── Package.swift
└── Sources
    └── ChatServer
        ├── ChatService.swift
        └── main.swift

Create a Package.swift file with the following content, substituting "x.x.x" with the latest releases of Kitura, HeliumLogger and Kitura-WebSocket-NIO:

// swift-tools-version:4.0
import PackageDescription

let package = Package(
    name: "ChatServer",
    dependencies: [
         .package(url: "", .upToNextMinor(from: "x.x.x")),
         .package(url: "", from: "x.x.x"),
         .package(url: "", from: "x.x.x")
    targets: [
            name: "ChatServer",
            dependencies: ["Kitura", "HeliumLogger", "Kitura-WebSocket"]),

The HeliumLogger package, while strictly not required, was added to enable logging.

Create a ChatService.swift file which contains:

// ChatServer is a very simple chat server

import Foundation

import KituraWebSocket

class ChatService: WebSocketService {

    private var connections = [String: WebSocketConnection]()

    public func connected(connection: WebSocketConnection) {
        connections[] = connection

    public func disconnected(connection: WebSocketConnection, reason: WebSocketCloseReasonCode) {

    public func received(message: Data, from: WebSocketConnection) {
        from.close(reason: .invalidDataType, description: "Chat-Server only accepts text messages")


    public func received(message: String, from: WebSocketConnection) {
        for (connectionId, connection) in connections {
            if connectionId != {
                connection.send(message: message)

The class has a Dictionary, connections, which is used to keep track of the connections of all of the connected clients. The Dictionary is maintained by the connected and disconnected functions, which are, respectively, adding and removing connections from the dictionary.

The received function, which receives binary messages, is rejecting the message, closing the client connection and removing the connection from the set of known connections.

Lastly, the received function, which receives text messages, simply echoes the message received to all clients except the one who sent the message.

It should be noted that all of these functions can be invoked from many threads simultaneously. In real applications, one should add locking around the access of non-thread safe artifacts of the application such as the connections Dictionary in this very simplistic example.

Create a main.swift file which contains:

// ChatServer is a very simple chat server

import Foundation

import KituraNet
import KituraWebSocket

import HeliumLogger
import LoggerAPI

// Using an implementation for a Logger

WebSocket.register(service: ChatService(), onPath: "chat")

class ChatServerDelegate: ServerDelegate {
    public func handle(request: ServerRequest, response: ServerResponse) {}

// Add HTTP Server to listen on port 8080
let server = HTTP.createServer()
server.delegate = ChatServerDelegate()

do {
    try server.listen(on: 8080)
} catch {
    Log.error("Error listening on port 8080: \(error).")

In the main.swift file:

  • The HeliumLogger is set up to log info, warning, and error type messages.
  • The ChatService defined in the ChatService.swift file is registered on the path chat.
  • An HTTP server is created and setup to listen on port 8080.

With this server set up clients should connect to the chat service as ws://host:8080/chat, where host is the host running the server.

The client

The client has a simple command line interface. At startup one passes the host and port number. The client simply reads messages to be sent from the terminal and displays messages received on the terminal as well.

You will need to create the client's directory structure to be something like this:

├── package.json
└── chat.js

Create a package.json file which, at a minimum, contains:

  "name": "chat",
  "description": "Simple chat server client",
  "version": "0.0.1",
  "engines": {
    "node": ">=0.8.0"
  "dependencies": {
    "websocket": "^1.0.23"

Create a chat.js file which contains:

/* main file of Simple Chat Server Client */

var readline = require('readline');
var WebSocketClient = require('websocket').client

var host = process.argv[2];

rl = readline.createInterface(process.stdin, process.stdout);

rl.setPrompt('> ');
var client = new WebSocketClient();

client.on('connectFailed', function(error) {
    console.log('Connect Error: ' + error.toString());

client.on('connect', function(connection) {
    connection.on('error', function(error) {
        console.log("Connection Error: " + error.toString());

    connection.on('close', function(reasonCode, description) {
        console.log('chat Connection Closed. Code=' + reasonCode + ' (' + description +')');

    connection.on('message', function(message) {
        if (message.type === 'utf8') {
            console.log('\r=> ' + message.utf8Data);

    rl.on('line', function(line) {

    rl.on('close', function() {
        console.log('Have a great day!');


client.connect("ws://" + host +"/chat", "chat");

Building and running the example

To build the server, in the server directory, type:

swift build

To run the server, in the same directory, type:


The server will now be up and running. The informational log message below will be displayed:

[INFO] [HTTPServer.swift:124 listen(on:)] Listening on port 8080

Setting up and running the client

To setup the client, in the client directory, simply:

npm install

That will install the websocket package.

To run the client, again in the client directory, run:

node chat.js host:8080

Where host is the hostname of the host on which the server is running, e.g. if your server is running on the localhost run:

node chat.js localhost:8080

As described above, the server echoes all text messages sent to it to all of the clients that have connected to it, with the exception of the client that sent the message. Therefore, in order to see the example in action you will need to connect more than one client to the server. The client can be run in several terminal windows on the same computer. You can then enter a message on one client and see it appear on another client and vice versa.

Example - Advanced

For a more complete example please see Kitura-Sample.

API Documentation

For more information visit our API reference.


We love to talk server-side Swift, and Kitura. Join our Slack to meet the team!


This library is licensed under Apache 2.0. Full license text is available in LICENSE.


  • Swift Tools 5.0.0
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Last updated: Wed Sep 20 2023 14:06:17 GMT-0900 (Hawaii-Aleutian Daylight Time)